1. Field of the Invention
This invention relates to a multi-channel optical receiver module and, in particular, to a multi-channel optical receiver module in which crosstalk can be reduced.
2. Description of the Related Art
Conventionally, communication signals of multiple channels which have different optical wavelengths are multiplexed to allow the propagation of the multi-channel communication signals in a single optical fiber.
Then, a multi-channel receiver module receives the multiplexed optical signal by itself.
As shown in FIG. 3, a multi-channel optical receiver module includes a receptacle 31 for leading light from an optical fiber (not shown) as a transmission path, a reflective member 32 facing to an optical path from the receptacle 31 at a predetermined angle, a reflective branching filter 33 facing to the reflective member 32 at a predetermined angle and for branching the optical signals, a line-type light-receiving element array 34 including multiple light-receiving elements for receiving the branched optical signals which are entering into the array with slightly different optical paths from each other, and a can-type receiver module 35 for mounting the light-receiving element array 34. Since electric power of a received light signal outputted from the light-receiving element (array 34) is weak, an amplifier IC chip (not shown) for amplifying the received light signal is incorporated inside of the can-type receiver module 35, so that an output pin 36 of the can-type receiver module 35 can output the signal amplified by the amplifier IC chip. An example of the amplifier IC chip may be a transimpedance amplifier to convert a received current into a voltage. The transimpedance of differential signal in the amplifier IC chip is 5 to 10 kΩ.
In addition, the multi-channel optical receiver module does not always exchange the multiplexed optical signal with only one transmission path. The multi-channel optical receiver module can exchange optical signals with multiple optical transmission paths, respectively. In this case, semiconductor members can be combined and integrated such that the line-type light-receiving element array 34 including the multiple light-receiving elements can be mounted on the can-type optical receiver module 35.
The related art to the invention is, for example, JP-A-8-116136.
An output signal from the amplifier is a differential signal. Essentially, the differential signal can prevent noise from being input or output when two output signal lines for the differential signal are disposed in parallel.
However, it is difficult to dispose in parallel the output signal lines for carrying the differential signal in the can-type receiver module. That is, it is difficult to make wires parallel since the output pins are disposed along the circumference of the can-type receiver module when the wires are provided to wire-bond linearly a differential output pin terminal of the amplifier IC chip to the output pin. Thus, it is difficult to prevent noise from being output from the wire due to the output of the amplifier IC chip in the can-type receiver module 35.
Within the can-type optical receiver module 35, received light output is transmitted through a signal line connecting the light-receiving element 34 and the amplifier IC chip. Electric power (i.e., received light output) in the line of the light-receiving element is typically very low, and there is a difference in electric power corresponding to the amplifier gain between the received light output and the output of the amplifier IC chip. Therefore, the former output is likely to be greatly affected by noise. That is, when one channel light-receiving element receives the noise from the differential output of another channel light-receiving element, crosstalk will occur. When the crosstalk occurs, a receiving sensitivity of the one channel light-receiving element is substantially reduced, thereby leading to errors.
It is noted that, although it may be possible technically to devise a pin arrangement such that the output pins are positioned to avoid noise output and input, such a pin arrangement is practically infeasible because of restriction of pin arrangement in fabrication.